Variable resistor



Feb. 10, 1959 M. E. BouRNs ET AL 2,873,337

VARIABLE RESISTOR Filed Nov. 14, 1955 2 Sheets-Sheet l AGENT Feb- 10,1959 M. E. BoURNs ET AL 2,873,337

VARIABLE RESISTOR Filed Nov. 14, 1955 2 Sheets-Sheet 2 /NVENTOQS BYraJJWm AGENT United States Patent() VARIABLE RESISTOR Marlan E. Bourns,Marvin E. Harrison, and Wilbur T. Hardison, Riverside, Calif.; saidHarrison and said Hardison assignors to Bourns Laboratories, Inc.

Application November 14, 1955, Serial No. 546,634

4 Claims. (Cl. 20162) nearly to the degree which the present variableresistor design permits. In addition, the present variable resistorsrequire the employment of either soldering or crimping techniques inorder to mount suitable end terminals or lugs to the end terminations ofthe resistive element employed.

Therefore, it is an object of the present invention to provide a new anduseful variable resistor.

It is a further object of the present invention to provide a new anduseful variable resistor which will be extremely compact, easy tofabricate and assemble, and inexpensive to manufacture.

Another object of the present invention is to provide a novel variableresistor in which the shorting bar used is closely adjacent theperiphery of the resistive element employed.

According to the present invention, an enclosure member orsemi-enclosure member provides a suitable mount for a preferredresistive element with a substantially oval or elliptically-shapedcross-section characterized by a small ratio of minor axis to majoraxis. Novel, snap-on terminal clips are mounted at the extremities ofthe re.- sistive element. A shorting bar is employed having aconfiguration which permits a portion thereof to lie substantiallyadjacent the periphery of the resistive element. An appropriateelectrical contact simultaneously engages the novel shorting bar and theexposed portion of the resistive element, and is linearly positioned byan appropriate lead screw.

The features of the present invention which are believed to be novel areset forth with particularity in the Vappended claims. The presentinvention, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may best be understood byreference to the following description, taken in connection with theaccompanying drawings, in which:

Figure 1 is a longitudinal sectional View taken through a variableresistor embodying the principles of the present invention;

Figure 2 is a top plan view of the same;` Figure 3 is a sectional view,taken along the line 3-3 in Figure l;

Figure 4 is a view similar to Figure 3, of another form of theinvention, showing a different type of contact member;

Figure 5 is a transverse sectional view, taken through the plasticmember which supports the resistance element and shorting bar, showinganother form of resistance element and shorting bar;

Figure 6 is a perspective view of a terminal clip which is adapted to besnapped onto the ends of the resistance element of Figure 1;

Figure 7 is a perspective view of a slidable contact member of the typeshown in Figure 1;

Figure 8 is a perspective view of the alternate form of slidable contactmember shown in Figure 4;

Figure 9 is a perspective view of the sheet metal housing of Figure 1,with the bottom portion thereof being in n up direction, and in itsextended condition;

Figure 10 is a view taken at 10-10 of the enclosure member of Figure 9,showing another form of bearing support for the lead screw;

Figure 11 is a perspective View of another modification of the contactmember, showing a novel manner of engaging the lead screw;

Figure 12 is a perspective view of an additional embodiment of avariable resistor employing a semi-enclosure member;

Figure 13 is a transverse sectional view of the device of Figure 12. y

ln Figure l, variable resistor 10 is shown to consist of a sheet metalhousing 11, having mounting tabs12 projecting from the ends thereof, alongitudinally extending lead screw 13, slidable contact member 14,insulating member 15, and resistance element 16. Resistance element 16is provided with terminal clips 17, and with a conductive strip, orshorting bar member 18, having its own terminal lug projecting from thehousing. The several configurations of slidable contact member 14,resistance element 16, terminal clips 17, and shorting bar member 18shall be explained later.

Figure 2 is a top plan view of the device in Figure 1, showing withparticularity how the terminal clips 17 and terminal lug of the shortingbar member 18 pass through suitable apertures in the insulating member15, and also through enlarged apertures in the sheet metal housing 11.These apertures in the housing 11 are of sucient width and length toprovide ample clearance for the terminal lugs, so that there is noelectrical connection therewith.

Figure 3 is a sectional view taken along line 3--3 in Figure 1, showingwith particularity that resistance element 16 has an oval or ellipticalcross section. Cone ceivably, however, resistance element 16 may haveother congurations, such as that of a rectangular bar having either orrounded corners.

The resistance element 16 comprises a central core 19, known as a card,upon which insulated resistance wire 20 is wound. Alternatively, theresistance element can take the form of a strip of composition material,such as bar 503 in Figure 5, which shall be shown and described later.

Figure 4 is a sectional view, similar to Figure 3, showing a modifiedform of the invention'. In this case, the contact member is generallyU-shaped, and one leg of the U hears on one edge of the resistanceelement, while the other leg wipes on rounded portion of the shortingbar, where the latter is wrapped around the other edge of the resistanceelement. lThis arrangement permits an extremely compact organization,and is advantageous for extreme miniaturization.

As hereinbefore explained, the resistance element 16 of Figure l maysimply comprise a resistive strip 503, manufactured from suitableresistance exhibiting composition, to which end terminals 18 may bemounted in tegrally therewith, during the molding process of lbar 503.The shorting bar is indicated at 502, and comprises Ya sheet metal stripwhich is bent to a ,U-shaped configura' 6,0410 accommodate theconnection of an electrical lead or other electrical element. The bentside portions l601 :and 602 and base portion 603 are shaped to provide asocket that receives the extremities of resistance element .-16 ofFigure 1. nwardly -directeddimples 605 areformed in portions 601, 602.,and 603, `which press against `and make electrical 4Contact withwindings of the wire 20. The slidable contact member of Figure 7includes a block member 700, having .a threaded laperture 7.08 providedtherein Yto receive the llead screw 12. The block ,member 700 also has acontact member 701, and means 702 to aix contact member 701 to blockmember 700. The securing means 702 may simply consist of a non- -roundprojection 702 molded in the top of the block 700, which passes througha correspondingly shaped hole 703 in the Contact 701. At the time thecontact 701 is assembled on the block 700, the projection 702 issoftened or melted and squashed down slightly to rivet the .contact tothe block. Such a forming process for the slidable contact shown inFigure 7 indicates that a thermoplastic material will be most suitablefor constituting block member '7 00. However, as is well known to theart, thermoplastics have appreciable coecients of expansion wherechanges lin ambient temperature occur, and without suitable correctivemeasures, the threaded aperture 70S wouid increase or decrease indiameter as .the plastic expands or contracts, causing the lead screw totit loosely within the threaded aperture, or to bind. To avoid thiscondition, we have provided a slot '706 between base portion 707 ofblock member 700 and the screw-threaded aperture 708, so that the latterwill remain substantially constant in diameter and therefore exhibit auniform frictio-nal force on the lead screw, des pite a considerablerange of expansion and contraction by the block member 700 due totemperature change.

T heslidable contact member, which is shown in Figure 8, is yanalternate form to that of Figure 7 and employs a block member 800,substantially equivalent to'block member 700 of Figure 7, but this timeprovided with a U-shaped contact member 301 which exhibits a wideseparation between portions 802 and 803 thereof, as contrasted to therather small separation between contact portions 709 and 710, ofbifurcated contact member 709. The purpose of the wide .separationbetween portions 802 ,and .803 shall be hereinafter explained in detail.

Figure 9 shows and describes enclosure member 11 in Figure l, only thistime in its upside down condition with bottom portion 900 open at,before being folded `down along the bend line 909. Enclosure 11 is shownprovided with apertures 901, 902, and 903 provided in .top yportion 904thereof. Apertures 901, 902, and 903 are of course provided toaccommodate terminal clips 17 and the terminal portion of shorting Abarmember 18, shown in Figure 1. Enclosure 11 is closed up by merelyrfolding the bottom portion 900 down along the bend line 190,9andsoldering the edges thereof .to the -adjoining edges 7of vthe sideand end walls. Bottom portion 9,00 may be `provided with a contouredretaining tab 905 having a 4semi-circular recess in the end thereofwhich ts into .and cooperates with van annular `groove `906 of the leadscrew 907, shown in dotted lines. The moved lend of @the lead screw Yis.dropped into .an v.opor-1 .notch 91.0 in :the end 9.11ct theenclosure., ,the bottom of which y,is .semi` .circular 1to receive the-greove 906 .of the `lead screw- 'Ihue .the notch 9.10am tab1905cooperate .to form a com- 4pleteenclosure, with :a circularholein theendA 911 through which `.the .Slotted end .of the lead screw *907projects tifnrzadiustmentzpurposes. aperture at the otherend endwisemovement.

of the enclosure, through which the other end of the lead screw passes,may be sealed with an 0-ring (not shown).

Figure 10 is a view taken along the line 10-10 in Figure 9, showing thatin lieu of the contoured retaining tab configuration of Figure 9, akeyhole slotted retaining tab 1000 may be provided at either end (or, ifdesired, both ends) of bottom portion 900 to receive an annular grooveprovided in the lead screw Asothat the lead screw need not pass throughthe ends of enclosure 11. The tab 1'000'is bent yback from the end ofthe bottom portion 900 `to provide clearance for the end of the leadscrew. This tab 1000, when lused in conjunction with the enclosureconstruction of Figure -9, but without .the open notch 910 of thelatter, makes it possible to hermetically seal the unit, inasmuch as'the ends of the lead screw are rrr tatably supported solely by the tabs1000 and are contained entirely within the connes of the enclosure 11.Thus, when the bottom portion 900 is soldered along its edges to theadjoining edges of the side and en dwalls, the enclosure 11 is sealedair tight.

With the above description of Figures 1 through 10, .it becomes possibleat this time to describe fully the operation of the variable resistor inits various embodiments. One of the more important characteristics ofthe invention is ease o-f assembly. The several elements of thecombination shown in Figure l are of course fabricated independently.Then resistance element 16, consisting either of a relatively at coreand a plurality of insulated Vwindings wound thereupon or simplyconstituting a b ar of appropriate cross section manufactured from aresistance composition, is provided with terminal `clips 1 7 at theextremities thereof (Shown in detail in Figure 6). Next, shorting barmember 18 is disposed in proximity with resistance element 16. SeevFigure 1 and Figure 3. In the case of a wire wound resistance elementthe sh0rting bar need only be disposed upon the insulation ofthe wiresthemselves to be insulated therefrom. Or, in the case of a resistancebar being employed, as shown in Figure 5, shorting bar 18 would need ybevspaced from the resistance `bar member 500. As a secondary approach,there might conceivably be disposed an air gap between the shorting barmember and `the resistive bar. In either case, that is whether theresistance element 16 o f Figure 1 is employed or Whether the resistivemember o f Figure 5 is used, the terminal projections are insertedthrough openings in the insulating member 15 and protrude throughapertures 901 and 903 of Figure 9. The insulating member 15 itself `maybe either glued to top portion 904 of enclosure 11 or may simply bepress-fitted withinv 'the enclosure 11 so as to engage vand thereby restupon top portion 904, or at least be in the proximity thereof. Theterminal projection kof the shorting bar member 18 will likewise beinserted through an aperture provided therefor (aperture 902 in Figure9) torprotrude from enclosure 11 (see Figure 1, Figure 2, and Figure 9).Lead screw 13 cooperating with slidable contact member 14 is nextmounted in position yand may either be retained by the cooperation ofeither the contoured retaining `tab of Figure 9 or the contouredretaining tab of Figure 10 with the annular groove thereof. In eithercase, the lead screw is rotatably supported on the housing and is heldagainst Bifurcated contact member 701 of slidable contact member 14 (seeFigure A7) simultaneously engages shorting bar member 18 andan exposedwinding of the resistance element 16, and this by means 0f portion 709and 710 of the slidable contact member shown in Figure '7. The slidablecontact member shown in 'Figure 7 may be used in conjunction with wirewound `resistance elements `and also'in conjunction with a'barresistivernem- '-'ber such as that shown in Figure -5. This is also trueof the slidable Contact member of Figure 8.

-The operation of the various embodiments shown and described in Figure'1 through Figure `10 and 'in the discussion relating thereto becomesapparent. 12012110.11 of `lead 'screw v13 causes -the progression in oneof two oppostte directions of slidable contact member 14. The locationof the slidable contact member 14 along the length of the resistanceelement determines the resistance between one of the terminal clips 17and the terminal portion of shorting bar 18. Conceivably of course, thedevice of Figure 1 might include only one end terminal, together withthe shorting bar member, for serving as a simple, two-terminal rheostat.

Figure ll describes a novel yet simplified contact member 1100 having apartially circular, internally threaded central member 1101 to engageand cooperate with lead screw 13. Upper end portions 1102 and 1103 areprovided to accomplish the simultaneous engagement with the shorting barand the resistance element. In the case of the utilization of contactmember 1100 in Figure 11, lead screw 13 would of course have to bemanufactured from an insulating material. The central member 1101 mayconsist of oppositely facing semi-circular straps struck out of thesheet metal ybase of the contact member 1100, which cooperate to form athreaded aperture for the lead screw.

Figures 12 Iand 13 show a variable resistor device incorporating asemi-enclosure member 1200, rather than the enclosure 11 of Figures 1and 2. The configuration and assembly of the variable resistance unit ofFigure 12 and Figure 13 is substantially the `same as that described inconnection with Figures l through however, in the case of the embodimentshown in Figure 12 and Figure 13, insulating member 15 is held in placeby the resilient cooperation therewith of L-shaped side portions 1201and 1202. The embodiment shown in Figure 12 and Figure 13 may be usedwhere hermetic sealing is not required.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects, and, therefore, the aim in theappended claims is to cover all such changes and modications as fallwithin the true spirit and scope of this invention.

We claim:

1. In a variable resistor, the combination of an elongated resistanceelement having one edge thereof rounded, a shorting bar of sheet metalcylindrically curved to conform to said one edge of said resistanceelement and overlying the same in close proximity thereto, said shortingbar being insulated from said resistance element, and a slidable contactwiping on an exposed portion of said resistance element and on saidshorting bar.

2. A variable resistor comprising a box-like enclosure, one wall of saidenclosure having a tab provided thereon adjacent one end of theenclosure, said tab being spaced from said one end and substantiallyparallel thereto, said tab having a keyhole slot formed therein, aresistance element mounted Within said enclosure, a lead screw disposedparallel to said resistance element, and a contact member mounted onsaid lead screw and wiping on said 6 resistance element, said contactmember being movable lengthwise of said resistance element by rotationof said lead screw, said lead screw being rotatably supported at one endby said tab and having a circumferential groove engaged by said keyholdslot.

3. A variable resistor comprising a body, an elongated resistanceelement mounted thereon, a shorting bar of sheet metal curved to conformto one edge of said element and overlying the same in close proximitythereto, said shorting bar being insulated from said element, the otheredge of said resistance element being exposed for electrical contact,and a contact member supported for movement along a path parallel tosaid resistance element and shorting bar, said contact member having twolaterally spaced upstanding lingers which wipe on said exposed portionof said resistance element and the oppositely facing surface of saidshorting bar.

4. In a variable resistor, an enclosure housing comprising four sidewalls and two end walls, three of said side walls and said end wallscoacting to form a box open on one side, the fourth side wall beinginitially open and being adapted to be bent `down along one edge toenclose said housing, said fourth side wall having a tab bent down fromone end thereof, said tab having a semi-circular recess in the endthereof, the adjacent end wall having an open notch formed therein witha semi-circular bottom, said notch and said tab being registered withone another when said fourth side wall is bent down to the closedposition to form oppositely facing halves of a bearing aperture in saidadjacent end wall, a lead screw having a circumferential groove formedtherein which is received within said bearing aperture, a contact memberdisposed within said enclosure housing and movable lengthwise thereof byrotation of said lead screw, and a resistance element within saidenclosure housing wiped by said contact member.

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